Mouth of the Appia (I.), in a cave under the Priorato or Priory of the knights of Malta on the Aventine, near the Marmorata and the Porta Trigemina. At the back of the cave, and connected with it by a natural tunnel, is a large reservoir of water in the heart of the hill, supplied by a natural spring, which was added to the water brought by the aqueduct for distribution. The latter part of this cave, behind the part here shewn, is under the garden of the monastery of S. Alessio, and the cave belonged to those monks. A plan and section of it is given in another plate. In the inner part of the cave is the natural spring, so that it is always knee-deep in water. This seems a likely place to have driven cattle into for concealment; there is no other natural cave under the Aventine, and this is close to the Porta Trigemina. The idea that there was a cave near the Forum Boarium seems to have arisen from a misunderstanding as to the exact site of the Porta Trigemina, which was supposed to be close to that Forum, instead of being a quarter of a mile from it, and close to the Sublician, or wooden bridge, where some remains of it have been found. The _specus_ of the Aqua Appia leads directly into this cave, and other aqueducts also meet there. The surplus water of the Trajan, far above this level, descends into it by a vertical pipe of terra cotta. Piranesi, in the last century, recognised the cave as the mouth of the Aqua Appia, and gives one of his admirable etchings of this part of the Aventine, with the cave at its foot. He was in advance of his time, but a great deal of fresh evidence has come to light since that period. PLATE V. I. AQUA APPIA. [Illustration: V. THE AQUEDUCTS.—APPIA. RESERVOIR IN THE GARDEN OF THE SESSORIUM, NOW OF S. CROCE, CALLED THERMÆ OF S. HELENA. A. SECTION. B. PLAN.]